linux-kernel - [PATCH v3 5/6] x86/uaccess: Avoid barrier_nospec() in 32-bit __get

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Message-ID: <967768924bd5295c231b6bfd37e3cbf85dad3cb2.1730166635.git.jpoimboe@kernel.org>
Date: Mon, 28 Oct 2024 18:56:18 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: x86@...nel.org
Cc: linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...en8.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
	Waiman Long <longman@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	linuxppc-dev@...ts.ozlabs.org,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Mark Rutland <mark.rutland@....com>,
	"Kirill A . Shutemov" <kirill@...temov.name>
Subject: [PATCH v3 5/6] x86/uaccess: Avoid barrier_nospec() in 32-bit __get_user()

The barrier_nospec() in 34-bit __get_user() is slow.  Instead use
pointer masking to force the user pointer to all 1's if the access_ok()
mispredicted true for an invalid address.

Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
---
 arch/x86/lib/getuser.S | 18 ------------------
 1 file changed, 18 deletions(-)

diff --git a/arch/x86/lib/getuser.S b/arch/x86/lib/getuser.S
index 5bce27670baa..7da4fc75eba9 100644
--- a/arch/x86/lib/getuser.S
+++ b/arch/x86/lib/getuser.S
@@ -35,8 +35,6 @@
 #include <asm/asm.h>
 #include <asm/smap.h>
 
-#define ASM_BARRIER_NOSPEC ALTERNATIVE "", "lfence", X86_FEATURE_LFENCE_RDTSC
-
 .macro mask_user_address size:req
 .if IS_ENABLED(CONFIG_X86_64)
 	movq $0x0123456789abcdef,%rdx
@@ -107,11 +105,7 @@ EXPORT_SYMBOL(__get_user_8)
 
 /* .. and the same for __get_user, just without the range checks */
 SYM_FUNC_START(__get_user_nocheck_1)
-#ifdef CONFIG_X86_64
 	mask_user_address size=1
-#else
-	ASM_BARRIER_NOSPEC
-#endif
 	ASM_STAC
 	UACCESS movzbl (%_ASM_AX),%edx
 	xor %eax,%eax
@@ -121,11 +115,7 @@ SYM_FUNC_END(__get_user_nocheck_1)
 EXPORT_SYMBOL(__get_user_nocheck_1)
 
 SYM_FUNC_START(__get_user_nocheck_2)
-#ifdef CONFIG_X86_64
 	mask_user_address size=2
-#else
-	ASM_BARRIER_NOSPEC
-#endif
 	ASM_STAC
 	UACCESS movzwl (%_ASM_AX),%edx
 	xor %eax,%eax
@@ -135,11 +125,7 @@ SYM_FUNC_END(__get_user_nocheck_2)
 EXPORT_SYMBOL(__get_user_nocheck_2)
 
 SYM_FUNC_START(__get_user_nocheck_4)
-#ifdef CONFIG_X86_64
 	mask_user_address size=4
-#else
-	ASM_BARRIER_NOSPEC
-#endif
 	ASM_STAC
 	UACCESS movl (%_ASM_AX),%edx
 	xor %eax,%eax
@@ -149,11 +135,7 @@ SYM_FUNC_END(__get_user_nocheck_4)
 EXPORT_SYMBOL(__get_user_nocheck_4)
 
 SYM_FUNC_START(__get_user_nocheck_8)
-#ifdef CONFIG_X86_64
 	mask_user_address size=8
-#else
-	ASM_BARRIER_NOSPEC
-#endif
 	ASM_STAC
 #ifdef CONFIG_X86_64
 	UACCESS movq (%_ASM_AX),%rdx
-- 
2.47.0